1. Field of the Invention
The present invention relates to an apparatus for recording information on and/or reproducing information from an optical disc which may an ordinary optical disc or a magnetooptic disc.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows a conventional apparatus for reproducing recorded information from an optical disc.
In FIG. 1, an optical head 30 applies a laser beam to an optical disc (not shown) which stores recorded information. The applied laser beam is reflected by the optical disc while being modulated by the recorded information, and detected by the optical head 30. A detected signal from the optical head 30 is processed by a arithmetic circuit 31, which produces a reproduced information signal and an focus error signal represented by an S-shaped characteristic curve. The focus error signal from the arithmetic circuit 31 is applied to an adder 60, by which it is added to a voltage from a rheostat 61. The rheostat 61 has one terminal connected to a positive power supply +B and another terminal to a negative power supply -B.
The adder 60 generates a sum output signal which is applied as a control voltage signal through a phase compensator 36 and a buffer amplifier 37 to a focus actuator 38. The focus actuator 38 moves the objective lens (not shown) of the optical head 30 along its optical axis over an interval depending on the voltage represented by the control voltage signal, until the laser beam emitted from the optical head 30 is sharply focused on the optical disc.
The optical head 30, the arithmetic circuit 31, and the focus actuator 38 jointly serve as an optical system, whereas the adder 60, the rheostat 61, the phase compensator 36, and the buffer amplifier 37 jointly constitute a signal processing system. The optical system and the signal processing system are generally mounted on different printed-circuit boards, respectively.
The conventional information reproducing apparatus shown in FIG. 1 require manual adjustments for higher focus servo accuracy. More specifically, while the information recorded on an optical disc is being detected by the optical head 30, the probe of an oscilloscope is held in contact with the output terminal of the arithmetic circuit 31 to display the waveform of the output signal thereof on the cathode-ray tube of the oscilloscope. Then, the rheostat 61 is adjusted so that the maximum amplitude of the displayed waveform is brought into synchronism with a certain comparative maximum amplitude.
The information reproducing apparatus shown in FIG. 1 suffers various shortcomings. The manual adjusting process required by the apparatus is tedious and time-consuming. Since the optical system and the signal processing system are mounted on the respective printed-circuit boards, if one of the systems is replaced with a new system, then the overall apparatus has to be manually adjusted again
When the optical head 30 is repaired or replaced, or the apparatus is otherwise subjected to characteristic changes or environmental changes such as temperature changes or the like, the offset voltage applied to the adder 60 varies. At this time, the initial setting of the rheostat 61 has to be changed in order to maintain a desired high level of focus servo accuracy.